Purple cabbage and turquoise poop

We recently got 7 hens and I have read that stringing up a cabbage to a string and letting them peck at it is a great way to keep them entertained and well fed. Shortly after they demolished the cabbage (in less that 48 hours- impressive) I noticed that their poop had this odd turquoise coloring to it. Has anyone else experienced this after their chickens have had a large purple cabbage intake?